In industry, one often needs to use fluid tight and/or gas tight pipes that also have a certain flexibility. In some cases, tightness problems can be overcome by using pipes made of a supple material, for example plastic, elastomer or equivalent, but very often these materials exhibit a certain gas permeability which may not be acceptable. It is also possible to use sections of a rigid metal pipe connected together by flexible joints. The tightness problem is then transferred to the joints of the connection.
It is possible to use corrugated metal pipes made from metal strips formed by rollers, spirally wound on a mandrel and continuously welded so as to form a tight pipe exhibiting corrugations, and therefore a flexibility increased by the shape of the corrugations. If the weld bead remains tight, the problem of the manufacture of a metal pipe that is notably perfectly gas tight and flexible thanks to the more or less corrugated shape of the generating lines is solved. But this manufacture is slow and requires a rather heavy manufacturing installation, and welding is a technical solution of delicate implementation and control. Furthermore, the bending fatigue strength is often decreased by welding and this manufacturing type only produces good results for certain types of metals.
It is also possible to form, by means of rollers, a cylindrical pipe slipped onto a mandrel of corrugated external shape. However, the drawback of this cold deformation is that is is rather slow and also that is requires relatively big machines, especially when the diameter of the pipe is of the order of about ten centimeters or more. This manufacturing is generally limited to relatively short sections.
The magnetoforming method is already used on elementary parts for performing deformations or joinings through crimping, welding or plating. This method can be performed by compression of the metal or on the contrary by expansion, according to the degree of deformation. But no solution is provided in the case of forming of the surface of a metal pipe that is several meters long.
The magnetoforming process is well known and will not be described here. It will just be reminded that it consists in sending a very short electric impulse in an electromagnetic coil located close to the walls of the part to be formed. The variation in the electromagnetic field produced by the coil generates, in the walls of the conducting metal pipe, an induced current which, by interaction with the current circulating in the coil (Laplace's law), exerts on the walls of the pipe forces equivalent to an electromagnetic pressure, said pressure deforming the pipe by pressing the walls against a forming die.